Theory of everything

The basic concept of the theory of everything is that one theory could explain every aspect of physics. The four basic forces of nature are gravity, the strong force, the weak force, and the electromagnetic force; the theory of general relativity explains gravity, and quantum mechanics explains the other three forces. Right now, no one theory that everyone accepts can explain all four forces.

String theory is a theory that could become the "theory of everything".

Unified field theory

Unified field theory is a hope for a theory of fields in physics that will explain all natural phenomena. Theories in physics can be very different and even disagree with each other. This is true for example for quantum mechanics (the science of very small things) and general relativity (the science of space and time). The goal of unified field theory is to take all the different parts of physics and combine them into one theory that explains everything. The Standard Model comes close to a unified field theory, combining everything except general relativity.